1. Field of the Invention
The present invention relates to a clock's out-of-synchronism state detection circuit and an optical receiving device using the circuit, and in particular, to a clock's out-of-synchronism state detection circuit for detecting an out-of-synchronism state of a clock signal with respect to a data signal in an optical receiving device, and an optical receiving device using the circuit.
2. Description of the Prior Art
At the time of occurrence of loss of a data signal to an optical receiving device and degradation in S/N thereof, the issuing of an alarm is an essential function to maintain the quality of a transmission line. As one of general methods of detecting the loss of a data signal and the S/N degradation, there is a method of using a frequency control voltage for a voltage-controlled oscillator used in a PLL (Phase Locked Loop) circuit, which performs clock regeneration. That is, at the time of occurrence of data loss and S/N degradation, the PLL circuit is greatly out of the locked state, and thus the frequency control voltage for the voltage-controlled oscillator therein greatly deviates from that when the PLL circuit is in a locked state. Thus, when this deviation of the frequency control voltage reaches to a predetermined value, the method can issue an alarm.
When S/N of a data signal to an optical receiving device is degraded, jitter in the data signal (frequency varies every one bit in the proximity of the inherent frequency of the data signal) increases and the signal approaches random noise. Then, the frequency control voltage greatly varies such that the voltage controlled oscillator of the PLL circuit may be pulled into its locked state.
On the other hand, even in the case in which predetermined quality of a transmission line is kept, jitter is actually included in the data signal more or less. In this case, the frequency control voltage also varies to pull the voltage-controlled oscillator of the PLL circuit into its locked state. Therefore, according to a method of using the frequency control voltage for the voltage-controlled oscillator in a conventional manner, it is difficult to set a threshold value for determining whether the operation is in a normal condition or in an alarm condition, and thus it becomes impossible to issue an alarm at the time of S/N degradation and loss of the data signal.